archimport: allow for old style branch and public tag names

archimport: allow for old style branch and public tag names

This patch adds the -o switch, which lets old trees tracked by
git-archmirror continue working with their old branch and tag names
to make life easier for people tracking your tree.

Private tags that are only used internally by git-archimport continue to be
new-style, and automatically converted upon first run.

[ ml: rebased to skip import overhaul ]

Signed-off-by:: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Martin Langhoff <martin@catalyst.net.nz>